Tetrapod Zoology

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I try not to under-estimate the intelligence of my readers, but couldn’t you be just a little more clueless? I mean, come on: virtually every person who left a comment realised that the ‘mystery animal’ from yesterday was a replica owl. Clearly, it was much, much easier than I thought. Anyway, well done everyone.

These owls are mostly based on Great horned owls Bubo virginianus, but the colour schemes are often a bit weird: the one I photographed has a red chest, though I doubt if this sort of thing makes much difference (hmm, or does it?). What’s amusing is that this particular decoy is at the top of Spinnaker Tower [shown below], a 170-metre-high visitor attraction located in the city of Portsmouth, UK. Spinnaker Tower was originally going to be called Millennium Tower, but construction didn’t start until 2001, and it wasn’t opened until 2005. It looks like the more famous Burj Al Arab in Dubai.

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Anyway, I assumed that the owl was put there to scare off pigeons, but realised later that gulls (Herring gulls Larus argentatus) were more likely to be the target. Big owls – and I’m thinking in particular of the European eagle owl B. bubo – can be significant predators of gulls, so it follows that gulls would avoid them… but would this only be the case in gull populations where the gulls have suffered from owl predation?

One thing I’d really like to know about these decoys is how effective they are: birds are predominantly visual animals, and you might assume that they quickly become unafraid of decoys that are sat in the same spot for week, months or years. I’ve certainly seen pigeons sitting, unconcerned, just a few metres away from one of these decoys, and you can find a few observations in the literature which indicate that the decoys are indeed ineffective if left in the same place for any period of time. A study on the damage made by woodpeckers to utility poles found that decoy owls are only effective for a very limited time, and Andrews (1961), Conover (1985) and Montevecchi & Maccarone (1987) reported that thrushes, crows, and Grey jays Perisoreus canadensis, respectively, became habituated to owl decoys over time. Marsh et al. (1992) and Liebezeit & George (2002) concluded that immobile owl decoys are mostly ineffective against corvids. Incidentally, I’m greatly intrigued by the title of Conover’s paper: ‘Protecting vegetables from crows using an animated crow-killing owl model’. [UPDATE: thanks to Cameron McCormick I now know that Conover tested the effectiveness of a model that looks like it’s killing a crow (see image below, from Conover (1985)). I was rather hoping that Conover was instead referring to a model owl equipped with motion-sensing lasers, heat-seeking missiles or tactical nukes, or perhaps to a robotic sentry gun disguised with feathers and a cute little owl face. Oh well]. Numerous anecdotes also indicate that the decoys are ineffective, and that pigeons, gulls, passerines and other birds will happily wander up to the decoys, sit on them, build nests on them, and so on…

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Mobile owl decoys – they flap their wings – are commercially available, and are presumably far more effective. One problem exists, and it detracts from their use on tourist attractions like Spinnaker Tower: they look hilarious.

Refs – –

Andrews, R. J. 1961. The motivational behaviour controlling the mobbing calls of the blackbird (Turdus merula). III. Changes in the intensity of mobbing due to changes in the effect of the owl on the progressive waning of mobbing. Behaviour 18, 161-176.

Conover, M. R. 1985. Protecting vegetables from crows using an animated crow-killing owl model. Journal of Wildlife Management 49, 643-645.

Liebezeit, J. R. & George, T. L. 2002. A summary of predation by corvids on threatened and endangered species in California and management recommendations to reduce corvid predation. California Department of Fish and Game, Species Conservation and Recovery Program Report 2002-02, Sacramento, CA.

Marsh, R. E., Erickson, W. A. & Salmon, T. P. 1992. Scarecrows and predator models for frightening birds from specific areas. In Borrecco, J. E. & Marsh, R. E. (eds) Proceedings of the 15th vertebrate pest conference. University of California, Davis, pp. 112-114.

Montevecchi, W. A. & Maccarone, A. D. 1987. Differential effects of a Great horned owl decoy on the behaviour of juvenile and adult Gray jays. Journal of Field Ornithology 58, 148-151.

Comments

  1. #1 The Ridger
    May 17, 2009

    Incidentally, I’m greatly intrigued by the title of Conover’s paper: ‘Protecting vegetables from crows using an animated crow-killing owl model’.

    I’m intrigued, too. Is there a missing hyphen, or has someone actually created a crow-killing model owl? Does it have lasers?

  2. #2 Rosel
    May 17, 2009

    If its any consolation I could only tell it was a bird.

    The Spinnaker tower looks very nice for a modern piece of architecture, in fact i’m quite jealous as Nottingham only has a piece of red metal junk called ‘Aspire’.

    I’d rather visit Portsmouth that Dubai. The roads in Dubai are terrible, the place is a building site, and all those grand skyscrapers are empty. /cynic

  3. #3 Rosel
    May 17, 2009

    In Duesseldorf it is common to see fake crows in the food market (and occasionally shops) to keep pigeons away.

  4. #4 Mo Hassan
    May 17, 2009

    This reminds me of when I was carrying out an animal behaviour experiment for a random compulsory module at uni. We were to observe the mobbing behaviour of rooks (Corvus frugilegus) in the presence of buzzards (Buteo buteo) and false owls. Annoyingly and rather amusingly, there was already a fake owl present outside a cottage where we were doing the experiment, so the rooks took no notice of our replica.

  5. #5 jude
    May 17, 2009

    You’ve got some dummies, but maybe we don’t comment as much. :)

  6. #6 David Kelly
    May 17, 2009

    Fakes have been used to fool birders too.

    There was a Black-crowned Night Heron (Nycticorax nycticorax) reported in Northumberland quite a few years ago which was present for a long time and twitched by many. Someone eventually got suspiscious and poked it with a long stick and it fell off its roosting branch. It was stuffed!

    David Kelly

  7. #7 The Ridger
    May 17, 2009

    That owl looks like it’s riding on the crow. I’d think killing would involve flapping of wings and grabbing beaks…

  8. #8 Jerzy
    May 17, 2009

    ‘crows using an animated crow-killing owl model’

    Protecting vegetables would be much more difficult, if the crows learned to use animated human-killing model. ;)

    I once saw Carrion Crow model put on a balcony of an unused apartment across the street. I must say, it took me a few days to wonder why this crow is always sitting the same way. ;) Anyway, pigeons perched just near it, but didn’t build a nest on a balcony, which probably was the point (the nest becomes incredibly dirty).

  9. #9 Jerzy
    May 17, 2009

    BTW – somewhere online there is a wonderful manual how to get rid of all American pests. If I remember well, model owl would spook corvids, but pigeons are famously insensitive to all models and noises.

    PS. Read this manual! It has sections of getting rid of, among others: snakes, shore larks, woodpeckers, bats, cougars, black, grizzly and polar bears. Incredible stuff!

  10. #10 Jerzy
    May 17, 2009

    Especially for you. No tetrapod entusiast should miss it: How to get rid of polar bears? :D

    http://www.icwdm.org/handbook/index.asp

  11. #11 Erik K
    May 17, 2009

    No tetrapod entusiast should miss it: How to get rid of polar bears? :D
    Jerzy

    Easy, raising a long-shafted broom towards it will do the trick!

  12. #12 DDeden
    May 17, 2009

    This might not work against polar bears, but maybe pigeons and gulls: mounted immobile plastic owl, with swivel neck, where the head slowly free-spins around due to the ear tufts (one reversed direction) catching breezes, like a propeller on a shaft. Optional gear: attach a small needle/cone tracking a groove on the rotating shaft, amplifying a quiet but clearly audible ‘hoot hoot…’ Very simple, cheap, adjustable for wind strength by bending the ears in or out.
    “An earfull aerfoil owl to fool the fowl”.

  13. #13 anon
    May 17, 2009

    “No tetrapod entusiast should miss it: How to get rid of polar bears?”
    Holy smoke. Thanks, Jerzy.

    I notice that “Swat bear on the nose with a rolled-up newspaper” is not suggested…

    But check this out – they’re showing how to shoot polar bears in various situations with “cracker shells” and “plastic shot” – presumably to scare bear off, but then they get to “lead slug”, range 0 to 30 yards (meters).

    I think that you really want to avoid situations in which a polar bear is within 30 meters, let alone 0 meters. And if you do take a shot at it, you want to make sure that your shot is very effective or that you’re located in a secure bunker.

  14. #14 Dartian
    May 18, 2009

    In many countries, hunters have traditionally used owl decoys to attract other birds, not to scare them away. The hunters are taking advantage of the fact that many birds will throw their normal caution aside and aggressively start mobbing the fake owl. This method is used particularly often to shoot corvids, but also occasionally gulls and diurnal raptors (which fortunately are nowadays protected by law, at least in most western countries).

  15. #15 Matt Mullenix
    May 18, 2009

    As a falconer I see two characteristics of birds that may relate to decoy effectiveness. One is that birds can certainly be mistaken in their conclusions, especially when they make quick decisions within established sequences of behavior. A trained hawk, for example, may react to a piece of black plastic flapping at a distance in the same way it would react to a starling or crow if these are habitual quarries for them. They may even go so far as to catch it.

    This is not to say hawks are stupid–they won’t continue to make this kind of mistake; and they generally make it in the first place based on an otherwise logical assumption (ie., the small black flapping thing at a distance is usually a starling).

    Birds’ second relevant feature is their fantastic eyesight. They see what we see and more, and better, and at a greater distance. While a crow may make a quick, faulty assumption about a crow-sized and shaped plastic effigy, it’s not because it can’t see the thing well enough to know the difference. Once it establishes that the model is not real, I doubt it would ever mistake it for a crow again.

    It might, however, tear it apart for nesting material or something. :-)

  16. #16 johannes
    May 18, 2009

    In Germany, replica storks and herons are quite popular. Those with more money than brains (see here:
    http://en.wikipedia.org/wiki/Dieter_Bohlen ) keep expensive Kois in open ponds, and herons eat them (the Kois, not the owners). In theory, a heron will consider a pond with a replica standing beside it occupied by a rival, and will stay away from this pond. This rarely works, herons are neither very territorial nor very stupid.

  17. #17 Darren Naish
    May 18, 2009

    And there are cases of real herons trying to court/mate with the plastic ones (they’re widely available here in the UK too). I’ve always wanted one, just for fun (though not in ‘that’ sense).

  18. #18 Jenny Islander
    May 18, 2009

    @anon, #17: I think that’s a dry way of saying that if the bear closes to within 30 meters, it’s probably planning to kill you, so switch to deadly force. A griz may (or may not) intend simply to beat you up and then walk off, hence the advice to curl up in a ball and play dead. Polar bears and blackies who seriously try to get closer are using predatory behaviors.

  19. #19 David Marjanović
    May 19, 2009

    Crows are supposed to scare pigeons away? How is that supposed to work? Here in Paris I often see them together.

  20. #20 DDeden
    May 19, 2009

    Re. bears: In many animals including bears (and gorillas), an upright stance and bipedal approach (typical of humans) is a dominance threat, and triggers a fight, flight or faint response.

    Getting down on the ground immediately, still, nullifies the threat posture, calms the bear, whether a female with vulnerable cubs or a competing adult male. But if the bear is truly hungry, either get the picnic basket or the big guns.

  21. #21 Monado
    May 24, 2009

    Crows and even grackles eat baby birds, so pigeons or other birds don’t like them around the nest.

    Maybe predators are not always the most useful signal. The traditional country way of keeping crows away was to nail a dead one to anything handy, so a model dead crow might be better than an owl. They make a tremendous fuss overhead–at least the first time they see it–but they don’t land and pull up your corn or whatever. The fuss is to teach all other crows that this is a bad neighborhood. However, I don’t know if they would continue mobbing or if they would learn to stay away. Konrad Lorenz wrote that he had to get rid of a pair of black swimming trunks before the jackdaws (small crows) that he was observing concluded that he was a jackdaw-murderer and taught all the others the same.

    We often have Canada Geese on any wide lawn, especially near water, defecating dark green feces wherever they go. At one place with wide lawns and decorative ponds, there was a decoy of a dead goose, “face down” with wings outspread in each pond–and no geese.

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